Deuterium (also referred to as hydrogen-2, symbol D or ²H) is a natural, stable isotope of hydrogen with a nucleus containing one proton and one neutron. δD analysis, also referred to as an isotopic signature, is used in conjunction with δ¹⁸O analysis to determine the V-SMOW (Vienna Standard Mean Ocean Water) value of water (H₂O) molecules. δD measurements can help inform ecology, hydrology, and paleontology studies.

Variations of δD and δ¹⁸O compositions in water are caused by isotope effects in the hydrological cycle due to isotope fractionation at all kind of phase transformation: evaporation, condensation, and others. Basically, the isotope fractionation is dependent of temperature. Therefore, δD and δ¹⁸O analysis from groundwater or ice core samples can be used to gain conclusions about the climatic conditions during groundwater recharge. 

Picarro has designed and produced several isotope analyzers that precisely measure δD in water for laboratory and field studies.